Modern computers are generally designed to receive expansion cards that add functionality to the computer. Such expansion cards may include, for example, a LAN network interface card, a wireless LAN card, a graphic accelerator card, etc., and are typically designed to be compatible with a given industry specification (e.g. mPCI, Cardbus, PC-card, etc.). These expansion cards are typically plugged in to the “host” computer and operate from the host's power supply or supplies. Certain industry specifications (e.g., the mPCI specification) presently require expansion cards to operate either from a main power supply or an auxiliary power supply (e.g., derived from a battery), which typically provide supply voltages of either 3.3V or 5.0V to the various circuits in the computer. In general, however, due to advances in integration technology and power management, modern integrated circuits (“ICs”) typically are designed to operate from a supply voltage of 3.3V (rather than 5.0V), and many are now designed to operate from a supply voltage of 1.5 V.
For these reasons, expansion cards conventionally include a power controller to select either a main or auxiliary supply voltage (which may be either 3.3V or 5.0V) from a host computer and convert the selected supply voltage to the voltages that are needed by the IC's on the expansion card. The power controller conventionally also functions as an on/off switch for the expansion card, so that the CPU in the host computer may shut down the expansion card as needed, e.g., to save power in a standby mode. It further conventionally includes a “bypass” circuit that is used to pass one of the supply voltages directly to the ICs on the expansion card without any voltage conversion, e.g., when the host voltage is so close to the voltages needed by the expansion card that voltage conversion is impossible. The power controller may also include circuits for monitoring the host main and auxiliary supply voltages and for sending a “reset” or shut-down signal to the ICs on the expansion card in the event of an overvoltage or undervoltage condition or in response to a RESET command from the host computer. Finally, the power controller may also include a standby supply circuit that provides power to certain circuits on the expansion card that remain active even when the expansion card is placed in standby mode (e.g., a wake-up circuit).
These features have conventionally been implemented via a custom-designed power controller circuit using a large number of discrete components and ICs. For example, a conventional power controller may require more than 28 discrete components, including a switching IC for on/off switching, a supply selection switch IC, one or more “main” DC/DC converter ICs having a linear regulator or a high-efficiency switched mode power supply (“SMPS”) converter, a “standby” supply DC/DC converter IC, and several supply monitoring and reset logic circuits including internal references, voltage comparators, time-delay circuits, etc.
FIG. 1 illustrates the manner in which supply selection and voltage conversion have been implemented in conventional expansion card power controllers. The host main and auxiliary supply voltages are received at terminals 102 and 100, respectively, and are connected to supply selection switch IC 108 (an SPDT-type switch) via terminals 104, 106. The selected output voltage at node 110 is then input to one or more DC/DC converter ICs 118. As shown in FIG. 1, the DC/DC converter ICs 118 are conventionally either switching-type converters (including two FET switches 114, 116, a pass inductor L1, and a shunt capacitor C1, as shown) or linear-drop-out regulators.
FIG. 2 provides a more detailed illustration of the conventional power controller circuit shown in FIG. 1. Supply selection switch IC 108 is conventionally an IC having two high-power, low-impedance FETs Q1 and Q2 and associated switching control circuitry. Switching transistors Q1 and Q2 are connected to the main supply voltage via IC pin 206 and the auxiliary supply voltage via IC pin 208, and their source terminals are connected together (at node 210) to IC pin 212.
DC/DC converter IC 114, as shown in FIG. 2, includes transistors Q3 and Q4, which operate essentially as switches that are either open or closed. Transistors Q3 and Q4 are controlled via control logic 220. The source terminal of transistor Q3 and the drain terminal of transistor Q4 are connected via IC pin 222 to series inductor L1. Inductor L1 in turn is connected to the output node 234, where the regulated voltage is output to the other circuits on the expansion card. Capacitor C1 is connected from node 236 to ground, in order to stabilize the output voltage against transients that the supply selection switch 108 and and any bypass circuitry (not shown) tend to create. The output voltage is taken at node 234 and also fed back via IC pin 224 to control logic 220.
As is known in the art, DC/DC converter IC 114 operates by switching the high-side power transistor Q3 in a pulse-width-modulated manner, while simultaneously switching the low-side transistor Q4 in an opposite fashion. In other words, when transistor Q3 is open, transistor Q4 is closed, and vice versa. As such, the source voltage at pin 216 is periodically connected to inductor L1 and capacitor C1. The voltage developed across capacitor C1 powers the load at node 234. In addition, the output voltage is typically sensed, such as by a voltage divider, and fed as one input to an error amplifier (in control logic 220). A reference voltage is fed to a second input of the error amplifier. The output of the error amplifier feeds one input of a comparator (also in control logic 220). The other comparator input is typically fed by a periodic control waveform, such as a triangle wave. The comparator, in turn, operates the power switch with a series of control pulses, the width of which are used to regulate the load voltage to the desired level despite fluctuations in the load.
In conventional expansion cards, additional power converters or linear regulator ICs (LDO1 and LDO2, not shown) may further be connected to IC pin 212 of the supply selection switch 108. These additional regulators may be used to provide additional supply voltages that may be needed by the circuits on the expansion card (e.g., 1.5 V).
It will be recognized that the conventional power controller described above is both complex and expensive. The power controller for each expansion card is conventionally custom designed. Although custom designs provide the benefit that the power controller can be optimized for a given expansion card's power requirements, the labor cost required to design a conventional power controller is very high. Because of this high labor cost and the cost of the numerous discrete components contained in the conventional power controller, the conventional power controller represents a substantial part of the overall cost of an expansion card. It would therefore be desirable to provide a power controller that could be integrated onto a single monolithic integrated circuit with a reduced number of components.